Electrolytic production of lead sulpho-chromates



March 3, 1942.

S. W..STOCKDALE ETAL ELECTROLYTIC PRODUCTION OF LEAD SULPH-CHROMATESFiled July 18, 1939 INVENToRs 67E/pfff muraria/uf 15557 E WfA VERATTORNEYS Patented Mar. 3, .1942

ELECTROLYTIC PRODUCTION OF LEAD SULPHO-CHROMATES Stephen W. Stockdale,Hammond, Ind., and Elbert F. Weaver, Calumet City, Ill., assignors toInternational Smelting and Refining Company, a corporation of MontanaApplication July 18, 1939, Serial No. 285,104

-3 Claims.

This invention relates to the production of sulpho-chromates, and isconcerned more particularly with the provision of a new and improvedmethod for vproducing lead sulphochromate pigments electrolytically. Thelead sulpho-chromate" products referred to herein are known as such inthe color industry and comprise complex salts of lead sulphate and leadchromate.

The method of the invention comprises passing an electric currentthrough a bifluid electrolytic cell comprising a lead anode, an anolytecontaining a substance capable of serving during electrolysis as asolvent for lead, a diaphragm, a catholyte comprising an aqueoussolution containing sulphate ions and chromate ions, and an insolublecathode. Sulpho-chromates, having the y desired chemical composition andphysical characteristics, are produced in accordance with this method bycontrolling the composition of the electrolyte as hereinafter more fullydescribed.

In carrying out a process embodying the new method, a lead anode isimmersed in an anolyte containing a substance capable of serving uponelectrolysis as a, solvent for lead. An aqueous solution ora substancecapable of yielding nitrate or acetate ions, such, for example, assodium nin trate or sodium acetate generally is preferred for use as theanolyte. An insoluble cathode (of iron, for example) is capable ofyielding sulphate and chromate ions such, for example, as sodiumsulphate and sodium chromate. The anolyte and catholyte are physicallyseparated by means of a permeable diaphragm which prevents the twosolutions from intermingling, but enables an electric current to passbetween anode and cathode and permits the migration of ions from oneelectrolyte to the other.

A biiiuiiA electrolytic cell of conventional design, equipped with oneor more diaphragms of the type usually employed in producing lead saltselectrolytically, may be employed in carrying out the process. Thecomplete apparatus preferably includes means for continuouslywithdrawing and circulating both the anolyte and the catholyte through acircuit outside the cell during the course of the electrolyticoperation. The lead sulpho-chromate product forms in the anolyte and maybe separated therefrom in any suitable immersed in acatholyte comprisingan aqueous solution of a substance' manner after withdrawal of theanolyte from the cell and before its return thereto. 'I'he sulphate andchromate concentrations in the catholyte are maintained at desiredvalues by the addition of sulphuric acid and chromic acid to thecatholyte at a point outside the cell.

In operating the cell to produce a lead sulphochrornate product, anelectric current is passed from the lead anode through the anolyte, the

diaphragm, and the catholyte to the cathode. Satisfactory operation issecured with a potential dilerence of about 3 to 5 volts between theanode and th cathode, and with a current density of Sodium sulphate andsodium chromate diffuse through the diaphragm from the catholyte intothe anolyte and there react with the lead nitrate to precipitate leadsulpho-chromate and to regenerate the sodium nitrate:

The tendency for the alkalinity of the catholyte to increase as a resultof the migration of sodium ions thereto and the production of sodiumhydroxideat the cathode is compensated for by the addition of sulphuricacid and chromic acid or their equivalents to the catholyte externallyof this cell:

heretofore known processes for producing sulphochromate pigments.

In some cases it is advantageous to maintain the catholyte at a slightlygreater hydrostatic head than the anolyte. In this manner any tendencyof the sodium nitrate to diffuse through the diaphragm from the anolyteinto the catholyte is diminished and at the same time diffusion ofsodium sulphate and sodium from the catholyte into the anolyte is aided.

The chemical composition and physical properties of the product of thecell may be varied considerably by varying the conditions prevailing inthe cell during electrolysis. 'I'he factors which exert particularinfluence in this direction and which should be properly controlled tosecure a lead sulpho-chromate product having optimum physical propertiesfor use as a pigment and to facilitate operation of the electrolyticcell are the following:

l. Concentration of salts in the electrolyte Concentration of sulphatein the anolyte should be maintained at about 0.075% to O.l% by WeightNazSO/i, and concentration oi chromate in the anolyte should bemaintained at about 0.675% to .l5% by weight 'liiazCrOra Corrosion oithe anode is facilitated by maintaining these concentration values, andaccumulation of soluble lead in the anolyte is thereby prevented.Soluble lead, if present in appreciable quantities in the anolyte, willcontaminate the cell product and render it unsuitable for use as apigment. The concentration of sulphate in the catholyte preferably ismaintained at about 1% to 3% by weight Na2SO4, and the concentration ofchromate in the catholyte preferably is maintained at about 2.5% to 3%by Weight NazCrUr. By maintaining these concentration values in thecatholyte, the rate of transfer of chromate and sulphate ions to theanolyte is easily established at a value to maintain proper ionconcentrations in the anolyte.

2. .alkalinity of the electrolyte .linity preferably is maintained byadding sulphurc acid and chromlc acid to the catholyte in suitableproportions and in suitable total amounts, as this procedure provides avery precise method of controlling the concentration of sodium chromateand sodium sulphate in the catholyte, either to maintain an establishedconcentration of these salts or to eifectv any desired increase orreduction in the concentration of either constituent.

3. Temperature of the electrolyte The temperature of the electrolytepreferably is maintained at about C. to 25 C.` Within this temperaturerange transition of the generally preferred light yellow rhombiccrystalsinto the darker yellow monoclinic crystals is inhibited.

A preferred process in accordance with the invention is illustrated inthe accompanying dlagrarnmatic flow sheet. An electrolytic cell tank I0is substantially llled with an anolyte l'l comprising an aqueoussolution of salt such as sodium nitrate or sodium acetate capable ofserving during electrolysis as a solvent for lead. For example, anaqueous solution containing 2% to 8% by weight of sodium nitrate may beemployed as the chromate anolyte. Immer-sed in the anolyte is a leadanode l2.

suitably spaced from the anode and also immersed in the anolyte is adiaphragm i3, which advantageously is in the form of a fabric bag.rli'he diaphragm bag i3 contains a catholyte lil comprising an aqueoussolution containing sodium sulphate, preferably in a concentrationbetween about 1% to 3% by Weight Na2SO4, and

sodium chromate, preferably in a concentration between about 2.5% and 3%by weight NazCrOr. As indicated above, the catholyte may be maintainedunder a slight hydrostatic head with respect to the anolyte. Aninsoluble cathode l5, for example a cathode of iron, is immersed in thecatholyte.

lin order to provide for circulating of catholyte through the catholytecompartment within the diaphragm bag i3, catholyte is continuouslyintroduced from a storage tank ld through a conduit l'l extending to apoint adjacent the bottom of the catholyte compartment. Catholyteoverflows continuously from the catholyte compartment through a spout(not shown) and passes through a conduit iii to a pump it, by means ofwhich it is returned to the catholyte storage tanlr it.

Sulphuric acid and chromic acid are added to the catholyte outside thecatholyte compartment (preferably to the catholyte in the storage tankifi) in. such relative proportions and in such total amounts as arerequired to maintain the desired concentrations of chromate and sulphatein the catholtye and to maintain the pH thereof at a proper value..

The cathode iii and the anode i2 are connected Y to s. suitable sourceoi electric current, and upon closing the electrical circuit thereactions described above take place with the formation of lead.sulpho-chromate in the arrolyte. The anolyte with the insoluble leadsulpho-chromate suspended therein is continuously withdrawn from thebottom of the cell tank it through a conduit The solid sulpho-chromateproductl is separated from the withdrawn anolyte in any suitable manner,ior example by a decantation operation or a filtration operation. Theseparated lead sulpho-chromate is dried in preparation for marketing,and the clarified anolyte is forced by a pump 2l to an anolyte storagetank 22. Anolyte from the storage tank is fed continuously through aconduit 23 into the anolyte compartment of the cell tank iii at a ratesumcient to maintain a constant level of anolyte therein;

is pointed out above, the anolyte necessarily contains a smallpercentage of dissolved chromate and dissolved sulphate which enters theanolyte from the catholyte chiefly by migration through the diaphragm i3during electrolysis. lf desired, in order to insure maintenance ofproper sulphateand chromate concentrations in the anolyte, smallquantities of catholyte may be introduced into the anolyte outside thecell, as by passing regulated small quantities of catholyte through aconduit 2t from the catholyte storage tank l@ to the anolyte storagetank 22.

The sulpho-chromate product of the cell may be produced in desired tonesof color from light yellow to lemon yellow, depending upon the relativeproportions of sulphate to chromate established and maintained in theanolyte. A product of light shade is obtained when a relatively highproportion of sulphate is present in the anolyte, and a product ofrelatively dark" shade is produced when the chromate concentration inthe anolyte is relatively high. Other factors, as indicated above, alsoinfluence the color and tone of the product.

We claim:

1. The method of producing a lead sulphochromate productelectrolyticallyY which comprises passing an electric current through abiiluid electrolytic cell comprising a lead anode, an anolyte containinga substance capable of serving during electrolysis as a solvent forlead, a diaphragm, a catholyte comprising an aqueous `solution of sodiumVsulphate and sodium chromate, and an insoluble cathode, and controllingthe chemical composition and physical characteristics of thesulpho-chromate product by maintaining in the catholyte sodium sulphatein an amount equal to about 1% to 3% by Weight Na2SO4 a-nd sodiumchromate in an amount equal to about 2.5% to 3% by weight Na2CrO4.

2. The method of producing a lead sulphochromate productelectrolytically which comprises passing an electric current through abiuid electrolytic cell comprising a lead anode, an anolyte containing asubstance capable of serving during electrolysis as a solvent for lead,a diaphragm, a catholyte comprising an aqueous solution of sodiumsulphate and sodium chromate, and an insoluble cathode, and controllingthe chemical composition and physical characteristics ofthe.sulphochromate product by maintaining in the anolyte sodium sulphatein an amount equal to about 0.075% to 0.15% by weight NazSOi' and sodiumchromate in an amount equal to about 0.075% to 0.15% by weight NazCrOi.

3. The method of producing a lead sulphochromate productelectrolytically which comprises passing an electric current through abiuid electrolytic cell comprising a lead anode, an anolyte containing asubstance capable of serving during electrolysis as a solvent for lead,a diaphragm, a catholyte comprising an aqueous solution of sodiumsulphate and sodium chromate, and an insoluble cathode, and addingsulphuric acid and chromic acid to the catholyte in such relativeproportions and in such total amount as to maintain in the catholyte aconcentration of sodium sulphate of about 1% to 3% by weight Na2SO4, aconcentration of sodium chromate of about 2.5% -to 3.0% by weightNa2CrO4, and a hydrogen ion concentration equivalent to a pH of about 7to 8.

STEPHEN W. STOCKDALE. ELBERT F. WEAVER.

